This paper presents a lumped-element circuit model for an Archimedean spiral inductor based resonant ion sensor. The system’s response to different concentrations of aqueous potassium chloride is measured, and the model parameters, which are based on physical phenomena, are extracted from measured data. The model parameters are extrapolated to obtain expressions that model the system’s response over a wide range of concentrations (1.66 mmol/L to 1 mol/L). The model helps generate insight into the behavior of the physical system, could be used to systematically optimize the sensor design, and could be extended to other analytes.